In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments, UEs, communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
It should be understood that “user equipment” is a non-limiting term which means any wireless terminal, Machine Type Communication (MTC) device or node e.g. Personal Digital Assistant (PDA), laptop, mobile, sensor, relay, mobile tablet or even a small base station communicating within respective cell.
The Radio Resource Control protocol, RRC, see 3GPP TS 36.331, is a signaling protocol for configuring, re-configuring and general connection handling in the LTE radio access network (E-UTRAN). RRC controls many functions such as connection setup, mobility, measurements, radio link failure and connection recovery. A UE in LTE can be in two RRC states: RRC_CONNECTED and RRC_IDLE. In RRC_CONNECTED, there is an RRC context established, that is, the parameters necessary for communication between a UE and the network are known to both entities.
Packet data traffic is often bursty, with occasional periods of transmission activity followed by periods of silence. To reduce the terminal power consumption, LTE has introduced a mechanism for continuous reception, DRX, which means that the UE monitors downlink control signaling transmitted in a Physical Downlink Control Channel, PDCCH, in only one subframe per DRX cycle, where a DRX cycle is defined as the duration of one ON-time period plus one OFF-time period. During remaining subframes of the DRX cycle, the UE has its receiver circuitry switched off.
Thus, the DRX mechanism is based on periods of activity, called OnDuration, where the UE must be awake and monitor the PDCCH. The onDuration time period is followed by a possible period of inactivity, called “Possible DRX” in FIG. 1. These DRX cycles can either be long or short DRX cycles as illustrated in FIG. 1. The RRC protocol activates the DRX mechanism of a given UE and defines the periods when the UE is in Active Time.
DRX in the RRC_CONNECTED state is described in section 5.7 of the 3GPP LTE MAC specification 3GPP TS 36.321. The purpose with DRX is to allow the UE some sleep time when it does not have to monitor the PDCCH channel for new transmissions. The time when the UE has to listen on the PDCCH channel is called Active Time, and when UE is not in Active Time it does not have to monitor the PDCCH channel. What is included in Active Time is defined in TS 36.321, section 5.7.
There are also DRX related timers and DRX related rules that may require the UE to be in Active Time during the “Possible DRX” time periods.
FIG. 1 shows both a long DRX cycle and a short DRX cycle, and the UE may be either in short or long DRX cycle at any given time, but not in both. There are specific rules for when the UE shall enter short DRX cycle mode, and when the UE shall enter long DRX cycle mode. Typically, when there is traffic ongoing in the system, the UE will be moved to short DRX cycle mode, and when there has been a time period of inactivity, the UE is moved from the short DRX cycle mode to long DRX cycle mode.
If a UE has been active with receiving or transmitting data in one subframe, it is likely that said UE will be scheduled again in the near future. Therefore, the UE will remain in active state for a configurable time after being scheduled. This is implemented by the UE starting or re-starting a DRX inactivity timer every time it is scheduled, i.e. successfully decodes a PDCCH for a first transmission. When the DRX inactivity timer expires, the UE is moved to short DRX cycle mode. The time during which the UE remains in the short DRX cycle mode is set by the timerdrxShortCycleTimer, which is associated with the short DRX cycle. As long as this timer is running the UE is in short DRX cycle mode. When the drxShortCycleTimer expires the UE is moved to long DRX cycle mode. The long DRX cycle is always a multiple of the short DRX cycle, so that the onDuration time period of the long DRX cycle will always coincide with the onDuration time period of one of the short DRX cycles. The number of short DRX cycles that will be needed to fill up one long DRX cycle is configured by the network. Note that the usage of the short DRX cycle is optional and if not configured by the network, the UE will always be in long DRX cycle mode, provided that the DRX function has been configured for the UE.
The purpose of having these two different cycles is to provide flexibility such that the UE is allowed to be more responsive by using short DRX cycle and to allow the UE to save battery by using long DRX cycle dependent on the current data rate of the UE and the current traffic load in the system.
MAC Control Elements
According to the LTE MAC specification 3GPP TS 36.321, layer 2 specific control information is provided in MAC Packet Data Units, PDUs, by including MAC control elements. A MAC control element is identified with a specific Logical Channel Identity, LCID, where a specific LCID value identifies a unique MAC control element. For downlink the reserved value range for available LCIDs to be used for identifying MAC control elements is 11 to 26, and for uplink the value range is 11 to 24. Hence, the LCID values that are available for new MAC control elements is a limited resource and it is important to not allocate these number if not absolutely needed.
The DRX Command MAC control element is used to order the UE to leave Active Time, during which time the UE is required to monitor PDCCH, and go to DRX inactivity, during which the UE is not required to monitor the PDCCH, and to use the short DRX cycle. Such DRX commands may be sent by the network, i.e. the eNB in LTE, when the network knows that the UE will not be scheduled in the near future, or when the likelihood that the UE will be scheduled in the near future is low. This case can for example occur after a handover of the UE when the UE is not reporting any data to transmit and there is no data in the downlink buffer in the eNB, then it is likely that the UE has no ongoing traffic. This means that a UE that is in Active Time will leave Active Time upon reception of the MAC control element and remain in a period of inactivity, DRX inactivity, until it is moved back into Active Time again. This may for example be due to the UE entering the onDuration period of the DRX cycle, or if the UE has data to transmit and sends a scheduling request to the network. It should be noted that if the UE is in long DRX cycle when receiving this MAC control element, the UE will be moved to short DRX cycle.
The DRX Command MAC control element is specified with a MAC subheader as illustrated in FIG. 2.
The LCID for this MAC control element is currently set to 11110 in binary format. There are two reserved bits (specified with ‘R’) currently set to 0, and one extension field which is set to 1 if more fields are present in the MAC header, and set to 0 if either a MAC SDU (Service Data Unit), a MAC control element or padding starts at the next byte (specified with ‘E’).